1. Field of The Invention
This invention relates to thin film resonators (TFRs) and a method of making TFRs.
2. Description of Related Art
TFRs are thin film acoustic devices which can resonate in the radio frequency to microwave range, for example, 0.5 to 5 Gigahertz (GHz), in response to an electrical signal. FIG. 1 shows a typical TFR 10 with a piezoelectric film 12 between a first electrode 14 and a second electrode 16 which apply an electric field to the piezoelectric film 12. The film 12 is made of a piezoelectric crystalline material, such as zinc oxide, aluminum nitride (AlN) and other piezoelectric crystalline material, which exhibits a piezoelectric effect. The piezoelectric effect occurs when the piezoelectric material expands or contracts in response to an electric field applied across the piezoelectric material, for example by the first and second electrodes 14 and 16, or produces an electric field in response to mechanical stress or strain applied to the piezoelectric material. If the electric field across the film 12 is an alternating electric field having frequency components corresponding to resonant frequencies (e.g., a fundamental frequency and harmonics), the fundamental frequency of which is defined for a film of uniform thickness as the acoustic velocity (v) in the film 12 divided by two (2) times the thickness (t) of the film or fr=v/2t. The film 12 will mechanically vibrate at the resonant frequencies which in turn produces an alternating electric field at the resonant frequencies.
The first and second electrodes 14 and 16 are typically of metal, such as aluminum. The acoustic impedance mismatch between the first electrode 14 and the air creates a first acoustic reflecting surface 18 at the interface between the top surface of the first electrode 14 and the air. A second acoustic reflecting surface 22 can be established at an interface between the second electrode 16 and a substrate 24 (or air if a portion of the substrate 24 under the film 12 is removed). Alternatively, acoustic reflecting layer(s) can be created between the second electrode 16 and the substrate 24 to suppress unwanted frequencies, such as harmonics of the fundamental frequency. The acoustic reflecting layer(s) can be formed from a material having desired characteristic acoustic impedance(s) and with the proper dimensions to provide desired reflection characteristics for the second reflecting surface at the interface between the second electrode 14 and the acoustic reflecting layers. As such, the acoustic reflecting layers can reflect desired frequencies while suppressing unwanted frequencies. An acoustic cavity created between the first and second reflecting surfaces and with the proper dimensions establishes a standing wave at the resonant frequencies of the piezoelectric film 12. The dimensions of the acoustic cavity, for example the thickness of the piezoelectric film 12 and the electrodes 14 and 16, define the operating frequencies for the TFR 10. Energy outside the operating frequencies of the TFR 10 is lost, while energy within the operating frequencies is preserved.
The TFR 10 structure can be formed on the substrate 24, such as a silicon (Si), Gallium Arsenide (GaAs) or other semiconductor substrate, for monolithic integration purposes, such as integration with active semiconductor devices. For discrete applications, the TFR 10 is typically formed on other suitable substrates, such as quartz, sapphire, aluminum nitride (AlN), or silicon carbide. If the TFR 10 has acoustic reflecting layer(s), the acoustic reflecting layer(s) are formed on the substrate 24 followed by the second electrode 16 which is formed on the reflecting layer(s). If there are no acoustic reflecting layers, then the second electrode 16 is formed on the substrate 24, for example using chemical vapor deposition (CVD) or sputtering. See, Kern and Vossen, xe2x80x9cThin Film Processes,xe2x80x9d Vols. I and II, Wiley and Sons. The piezoelectric film 12 is then formed on the second electrode 16, and the first electrode 14 is formed on top of the piezoelectric film 12, for example using chemical vapor deposition (CVD) or sputtering. To improve the performance of the TFR 10, a portion of the substrate 24 is removed from under the second reflecting surface 22. To remove the portion of the substrate 24, the substrate 24 includes an etch stop 28, such as a boron doped p+ layer implanted in a silicon (Si) substrate, at the upper surface of the substrate 24 adjacent to the bottom of the second electrode 16. The etch stop 28 is used to protect the second electrode 16 from a chemical etch removing the portion 30 of the substrate 24.
By growing the piezoelectric film 12 on the second electrode, the resulting piezoelectric film 12 is polycrystalline in that distinct crystals having different lattice orientations are present throughout the piezoelectric film 12. Such a non-uniform or irregular crystalline structure with grain boundaries between the differently oriented crystallites or crystal grains reduces the quality of the piezoelectric film 12.
Two figures of merit are used to measure the quality of piezoelectric films: a quality factor Q and an electro-mechanical coupling coefficient. The quality factor Q for a TFR is a measure of the resonance quality of the acoustic cavity while the coupling coefficient is a measure of the efficiency of conversion between electrical and mechanical energy within the acoustic cavity. Both of these figures of merit are inversely proportional to the acoustic loss introduced by the TFR at the operating frequency band. If the piezoelectric film 10 has a polycrystalline structure with grain boundaries and other defects, such as point imperfections or dislocations in the crystal lattice, or poor reflectivity of the reflecting surfaces 18 and 22 for example due to surface roughness, acoustic losses can result from acoustic scattering within the film 12 and acoustic radiation into the surrounding areas of the device 10. Thus, if the film 12 is polycrystalline, acoustic losses will be introduced by the film 12, thereby producing a lower quality TPR.
TFRs can be used at radio frequency (RF) because piezoelectric films can be made thin, for example at higher frequencies, such as 0.5-10 GHz, the piezoelectric film 12 can be between 0.4 and 8 microns in width. Because TFRs produce an alternating electric field at the resonant frequency in response to an alternating electric field having frequency components corresponding to the resonant frequencies, TFRs can be used as radio frequency (RF) filter elements. TFR filters have a distinct size advantage over conventional RF filters, such as those based on ceramics. For example, thin film resonators can have volumes of 1.5 cubic millimeters while ceramic resonators are typically not less than hundreds of cubic millimeters in volume. At the same time, a ceramic element typically introduces more loss to the input signal at the operating frequency band than the TFR. TFR also have higher power handling capabilities than surface acoustic wave (SAW) devices, for example 200 milliwatts vs. 2 watts. As mentioned above, however, TFRs can introduce losses to an electrical signal applied to the TFR in part due to the polycrystalline structure of the film 12. Typical TFR fabricating methods produce piezoelectric films with on the order of 108 distinct crystalline orientations separated by grain boundaries.
Thus, a need exists for a high quality TFR which introduces low loss to the electrical signal applied to the TFR.
The present invention involves a thin film resonator (TFR) produced with an improved piezoelectric film which is epitaxially grown on a growing surface, resulting in a piezoelectric film with less grain boundaries. Epitaxial growth refers to the piezoelectric film having a crystallographic orientation taken from or emulating the crystallographic orientation of a single crystal substrate or growing surface. For example, by epitaxially growing a piezoelectric film on a single crystal silicon substrate as the growing surface, an improved piezoelectric film is produced with little or no grain boundaries. In accordance with another aspect of the present invention, a method of making a TFR is disclosed in which the piezoelectric film is grown on a substrate. Subsequently, a portion of the substrate is removed, and the electrodes are deposited on either side of the piezoelectric film.